Before Jonas Salk’s polio vaccine was widely introduced in 1955, there were an average of 35,000 cases of Poliomyelitis each year in the United States alone. Through dedicated vaccination efforts, there were only 416 documented cases around the world in 2013. Despite being so close to global eradication, there was a massive outbreak in the Democratic Republic of Congo (DRC) in 2010 that ended up killing 47% of the 445 individuals who were affected.
At the time, the outbreak’s severity was initially presumed to be due to low vaccination rates in the region. Upon further investigation, it became clear that the vaccination rate was high enough to where it should not have been as prevalent as it was, particularly in adults. Poliovirus is typically passed through contact with an infected patient’s feces, so overpopulation and lack of clean water access could have contributed to the epidemic.
Earlier this year, researchers analyzed the virus’s genome and found that they were dealing with a mutated strain. It turned out to be very closely related to a mutant strain identified in Angola in 2009. Jan Felix Drexler of the University of Bonn was lead author of the paper, which was published in Proceedings of the National Academy of Sciences.
Scientists have now discovered the truth of why the DRC epidemic was so deadly: the strain of poliovirus is resistant to vaccines. The strain contained two mutations in the protective capsid which helped the virus escape antibodies from the immune system.
The researchers took blood samples from over 60 vaccinated volunteers from Germany as well as DRC's neighbor, Gabon. The blood was then exposed to various strains of poliovirus, with some fairly unsettling results. The antibodies did not work as well when confronted with the mutant strain responsible for the 2010 outbreak. In fact, 15-30% of the vaccinated volunteers would have been vulnerable if they had been exposed to the virus.
Drexler’s team cautions that there could be other mutated strains out there, so health officials need to be vigilant. Additionally, though inoculation with previous polio vaccines provides outstanding protection against the most common strains of the virus, vaccinated individuals could still become infected with a new, mutated strain. They recommend getting revaccinated in order to be better protected.
Through the hard work of dedicated health care workers, polio is now endemic in only Afghanistan, Pakistan, and Nigeria. Afghanistan’s rise in polio cases unfortunately correlates with the number of attacks on health officials working in the country to eradicate disease.
However, DRC’s outbreak and recent outbreaks in China and Tajikistan have demonstrated that infection can occur outside of these countries. The discovery of a polio strain that resists vaccination could definitely complicate these efforts. Global eradication of the virus is very close, but as long as it is out there, it will continue to pose a threat.